The invention relates to a solar cell, having an integral protective diode which has a polarity opposite the solar cell and includes at least one diode semiconductor layer, and the solar cell has at least one back contact, a substrate as a carrier for photoactive layers that include at least one front layer and one layer toward the substrate of different polarities, and a front contact, and the protective diode is integrated in the solar cell and disposed on the front side of the solar cell. The invention also relates to a method for producing a solar cell, having an integral protective diode which has a polarity opposite the solar cell and includes at least one diode semiconductor layer, and the solar cell has at least one back contact, a substrate as a carrier for photoactive layers that include at least one front layer and one layer toward the substrate of different polarities, and a front contact, and layers are formed in particular by epitaxial. growth.
In large-area diodes with areas of a plurality of square centimeters of the kind that solar cells of semiconductor material with at least one p-n junction form, micro-short circuits, that is, local, small-sized electrical (ohmic) connections via the p-n junction of the semiconductor material, are often unavoidable. They arise for instance from surface damage during substrate production or from dopant accumulations, for instance at crystal defects such as dislocations, in particular in the formation of epitaxially formed p-n junctions, for instance in solar cells made from the elements of groups III-IV of the periodic system.
The thus-created micro-short circuits admittedly impede the function of the diode as a solar cell to only a slight extent if at all in the flow direction. However, when the cell is operated in the depletion direction, the defects can cause the destruction of the cell. For instance, if a plurality of solar cells or solar generators are connected in series in a so-called string to form a solar array, then when the p-n junction of a generator is blocking--which can happen if it is in shadow--the solar current is forced through the ohmic micro-short circuits by the high string voltage of the remaining solar cells or generators that are still in the light. This can lead to severe local heating, redoping that produces low impedance, or in other words local severe denaturing of the semiconductor, and finally the destruction of the cell itself.
To avoid such local severe heating or so-called hot spots, it is known in series-connected solar cells to dispose protective diodes parallel to the solar cells, with a flow direction extending counter to that of the solar cells.
In solar cell strings, that is, a group of series-connected solar cells or solar cell modules, the individual solar cells can be protected with integral protective diodes for the sake of achieving greater reliability and avoiding the failure of entire strings (see Lippold, Trogisch, Friedrich: Solartechnik [Solar Technology], Berlin, Ernst, Verlag fur Architektur u. Techn. Wiss. [Ernst, Publishing House for Architecture and Engineering] 1984, pages 265, 266). However, the additional expense for internesting to form strings of suitable integral protective diodes that are provided on the backsides of the individual solar cells is considerable, since in addition electrically conductive connectors must be applied to the front side of the solar cells.
A solar cell of the type defined at the outset can be found in German Patent Disclosure DE 38 26 721 A1. For forming the protective diode, it is necessary first to apply an additional layer of semiconducting material to the photoactive layer; this additional layer is then regionally etched away.
From the United States professional journal article J. M., Olson et al, "A 27.3% efficient Ga.sub.0.5 In.sub.0.5 P/GaAs tandem solar cell", Appl. Phys. Lett. 56 (7), 1990, pages 623-625, a cascade solar cell is known in which a tunnel diode extends between pairs of photoactive layers that form photoactive cells.
A p-i-n solar cell with a Schottky protective diode can be found in European Patent Disclosure EP 0 327 023 A1.